adverb
how dare ...; how could ...
Expresses indignation or astonishment at someone's audacity. Often used in rhetorical questions or exclamations.
よくもそんなことが言えるね。
How dare you say such a thing!
よくもまあ、あんな嘘をついたものだ。
How could he tell such a lie!
adverb
Marks an outcome as contrary to expectation, often with a nuance of admiration or mild surprise. Less common than the indignant sense.
よくもこんなに早く終わらせたね。
You finished it surprisingly quickly.